With the verb ‘be’ in the past and future tenses, nominative case (Я был бедня́к) is also possible though rarely used with nouns, and in the present tense, nominative is the only variant (Я бедня́к, not *Я бедняко́м). Other verbs prefer instrumental in all tenses.

The adjectives behave the same way but nominative is more widely used with the verb ‘be’ in the past and future.

(*Стена́ была́ зелена́ for ‘The wall was green’, however, sounds terribly wrong to my ears, and I have no freaking idea why
The short forms are frequent with participles. With adjectives, they sound bookish or not used at all in some cases.)